1. Field of the Invention
The present invention relates to a resistance measuring apparatus that measures the resistance of a circuit to be measured.
2. Description of the Related Art
As one example of this type of resistance measuring apparatus, the resistance measuring apparatus disclosed in Japanese Examined Patent Application Publication No. H02-7031 (see Paragraphs 1 to 4 and FIG. 2) is known. This resistance measuring apparatus includes an injection transformer that is clipped onto a connecting lead of a measured network and injects a current of a second frequency, which can be discriminated from a current of a first frequency that flows in the measured network (“measured object”), into the measured network, a detection transformer (“detection coil”) that is clipped onto the connecting lead and detects the above two types of current flowing in the measured network, a frequency selecting circuit (more specifically, a frequency-selecting amplification circuit) that extracts the second frequency component out of the output of the detection transformer, a rectifying amplification circuit that rectifies and amplifies a signal (voltage) outputted from the frequency selecting circuit (frequency-selecting amplification circuit), and display means (more specifically, an indicator) that is driven by the signal (voltage) outputted from the rectifying amplification circuit and displays the output of the rectifying amplification circuit. In addition, the injection transformer is equipped with a feedback loop that includes an injection coil, which is supplied with an output voltage of an oscillator and injects the current with the second frequency into the measured network, and a feedback coil and is capable of changing the voltage supplied to the injection coil so as to make the voltage induced in the feedback coil constant. In this resistance measuring apparatus, since control is also carried out to make constant the injection voltage obtained by dividing the voltage induced in the feedback coil by the ratio of the number of windings in the feedback coil to the number of connecting leads (the number of leads clipped onto, which is one in this example) of the measured network, by detecting a voltage generated across a resistor connected to the detection transformer due to the current flowing in the detection transformer, it is possible to measure the resistance value of a resistance element (i.e., a resistance to be measured) connected to the measured network based on the resistance value of the resistor connected to the detection transformer, the voltage generated across such resistor, the voltage generated across the feedback coil, the number of windings in the injection transformer (injection coil), and the number of windings in the detection transformer (detection coil).